Essential magnitudes describing the quality and properties of concrete mixture are the yield strength, toughness and air content of the mass, the yield strength being often described by means of slump. By using these magnitudes, the rheology of the concrete mass can be determined, which is of use for manufacturing concrete mass of suitable type for different casting processes and products.
The slump and air content of the concrete mass is measured traditionally by means of sampling from a concrete mass batch. For the measurement of the slump, generally a vessel (slump cone) is used, with open bottom and open top, conically narrowing upwards, to be located on a floor or on some other suitable surface. The slump cone is filled with fresh concrete mass in several batches, and between the adding of the batches, the concrete mass in the slump cone is compacted manually. After being filled up to its upper surface and compacted, the conical vessel around the concrete mass is removed from around the concrete mass, whereby the concrete mass is free to spread on its base. After that the difference between the height of the upper lever of the spread concrete mass heap and the initial situation, in other words, the height of the upper surface of the filled slump cone will be measured. This difference stands for the slump of the concrete mass.
Concrete rheometer is used for measuring the toughness of the concrete in research on the field of concrete.
The air content of the concrete mass is measured with a separate measuring device of the air content, said device being filled with a certain amount of fresh concrete mass and, after that, sealed and measured. The construction of the device, as well as the measuring method, is defined in the international standards of the field of concrete.
The methods used for measuring the slump and air content of concrete mass as described above, are slow, time consuming and difficult. In addition, the concrete rheometer necessary for the toughness measurement is expensive.
Attempts have been made to describe the properties of the concrete mass also by means of the power taken by the mixer. This power or moment of the normal mixing speed alone, however, does not describe the workability of the concrete with an adequate accuracy, because the mass flow in the casting process of the concrete takes place with low speed.
One way to determine and regulate the properties and workability of the concrete mass is to define the moisture content of the concrete mass. Thereby the properties of the concrete mass can be conditioned by adding either liquid or dry substances to the mix. This kind of a solution utilizing the measurement of moisture content, however, is not adequate or satisfactory, especially for the concrete masses, in which agents are used. In these masses, the combined effects of the agent and the concrete can lead to changes in the workability properties of the concrete, even if the moisture content of the mass is unchanged.
The patent publication U.S. Pat. No. 6,227,039 discloses a system and a method for controlling the concrete manufacturing, wherein the rheological properties of the concrete mass are measured and controlled. In the solution in accordance with said publication, a concrete mass is manufactured, and after that its workability and/or other magnitudes describing the necessary properties of the concrete mass are defined with a separate measuring device, and the manufacturing of the concrete mass will be controlled based on these test results.